1. Dark Current in ILC Main Linac N.Solyak, A.Sukhanov, I.Tropin ALCW2015, Apr.23, 2015, KEK LCWS'15, Tsukuba, 04/2015Nikolay Solyak1

2. Motivation Halo particle and Dark current in Main Linac produce radiation, which affect: –Beamline components and cables inside of CM –electronics outside of CM in Main Linac section of tunnel –Electronics and personnel working in service part of tunnel Extensive studies are required to investigate halo & dark current radiation. –To protect the beamline elements from sever damage. –Reduce thickness and cost of the radiation shield. LCWS'15, Tsukuba, 04/2015 Nikolay Solyak 2 Thickness of wall (1.5-3.5m) separating service and operational facility is determined by max beam losses in tunnel Cross section of Kamaboko tunnel Recent studies of dark current: TESLA. V.Balandin et.al, http://tesla.desy.de/new_pages/TESLA_Reports/2003/pdf_files/tesla2003-10.pdf) http://tesla.desy.de/new_pages/TESLA_Reports/ Solyak et. al.,Dark current midell for ILC MainLinac*, EPAC2008, https://accelconf.web.cern.ch/accelconf/e08/papers/mopp036.pdf https://accelconf.web.cern.ch/accelconf/e08/papers/mopp036.pdf T.Sanami, KEK, ALCW2014 Belgrad,. LCLS-II, M.Santana, DOE review Dec.2014 JLAB 12GeV upgrade

3. MARS simulation MARS simulations is time consuming. To minimize simulations and understand average and maximal radiation conditions we formulated 3 Tasks: Task #1 – Losses inside individual cavityTask #1 – Losses inside individual cavity Simulate radiation from particles lost inside the same cavity where they were generated. This level of radiation (normalized) will be same for each cavity. Task #2 – Losses from survived dark current particles generated by single cavityTask #2 – Losses from survived dark current particles generated by single cavity Assume that only one cavity in each RF unit (3CM) contributes to dark current. In worst case this cavity is located next to the magnet. Survived particles will be accelerated in all 3 CM (26 cavities) or longer, before get defocused and lost in next quads Task #3 – Losses from dark current generated by all cavities.Task #3 – Losses from dark current generated by all cavities. All cavities generate dark current. File of lost particles along 4 CM’s is generated by tracking code and use as input for MARS Note: All survived particle leaving cavity particles are tracked through the chain of cavities and magnets until they will be lost ion the wall. Results are saved in file and used in MARS as input file. LCWS'15, Tsukuba, 04/2015 Nikolay Solyak 3

4. MARS model for ML (T.Sanami) LCWS'15, Tsukuba, 04/2015 Nikolay Solyak 4 Sanami model extended to 4 CM’s (DKS lattice) 4Q4 + 9cav + 9cav + 4Q4

5. MARS geometry (cont.) LCWS'15, Tsukuba, 04/2015 Nikolay Solyak 5 Cavity geometry Quadrupole magnet in CM CM details in MARS real cavity (red) vs. MARS model

6. Splittable ILC quadrupole in MARS model LCWS'15, Tsukuba, 04/2015 Nikolay Solyak 6

7. Field emission model LCWS'15, Tsukuba, 04/2015 Nikolay Solyak 7

8. Dark current model Particle generated in each cavity are divided in 3 groups (files). Each group is treated in MARS separately : –Particles lost inside cavity –Survive particles, leaving cavity from left beam-pipe (opposite to main beam) –Survive particles, leaving cavity from right beam-pipe Particles lost inside the cavity (>90% of total generated particles) was used in MARS as a input source for Task#1. In ILC distance between cavities 5.75 λ (+3λ between CM’s)  dark current can be trapped in acceleration only in one direction. Survive particles leaving cavity in opposite direction is not simulated by MARS. Survived particle moving in direction of main beam are tracked through all cavities and magnets until they will lost at the surface. File is used as source for Task#2 and 3 Each particle has weight defined by NF field emission model. Normalization is done by summarizing dark current at the end of CM (50nA). LCWS'15, Tsukuba, 04/2015 Nikolay Solyak 8

9. Particle tracking Distribution of particles in cavity #5 9 N Sum of FN weights Absorbed in cavity 9023992.2 % Exit to the left 48853.78 % Exit to the right 48763.99 % Survived (4%) particles track through the string of 25 cavities and Q2 FE is simulated as the following:FE is simulated as the following: -100k electrons are simulated -uniform distribution of emitted electrons in Z and RF phase 63 MV/m (gradient 31,=.5MV/m) -FN weight is calculated ( β=100, E s_max = = 63 MV/m (gradient 31,=.5MV/m) -only electrons with the normalized weight > 0.01 are selected for tracking

10. Trajectories 100 trajectories with weight > 0.01 10 100k emitters random in phase and location with weight > 0.01 green - lost in cavity, red - exit to the right, blue - exit to the left

11. Location and energy of the particles lost in cavity LCWS'15, Tsukuba, 04/2015 Nikolay Solyak 11 Generated

12. Spectrum of the particles lost inside cavity LCWS'15, Tsukuba, 04/2015 Nikolay Solyak 12 Lost in same cell 2cells 3cells 4cells Acceleration: ~ 3 MeV/cell

13. Losses in cavity - energy distribution Black - no weight, red - each track taken with weight 13 Particles with higher energies have typically higher weight

14. LCWS'15, Tsukuba, 04/2015 Nikolay Solyak 14 Energy distribution of survived particles

15. Particles leaving cavity (L = 1327 mm including beam pipes) R’ vs R 15 Color = energy Energy distribution at last iris

16. MARS tracking results for particle lost in cavity Task#1 result Radiation outside of the CM well below safety regulation limits. Only few tracks comes from high energy particles, but level of radiation is low. Need more statistics, especially in high energy tail to plot map of radiation inside and outside CM (work in progress). LCWS'15, Tsukuba, 04/2015 Nikolay Solyak 16

17. Tracking of survived particles through the cavity string LCWS'15, Tsukuba, 04/2015 Nikolay Solyak 17

18. Losses in 3 CM’s (between nearest quads) LCWS'15, Tsukuba, 04/2015 Nikolay Solyak 18 100K particle generated in 1 st cavity, 4% survived and tracked thru 3CM’s 50% of them lost in chain of cavities, the rest will reach quad. Most particles are lost at diaphragms. Decelerated particle can stop and kicked to the wall

19. Lost particles between magnets: Energy LCWS'15, Tsukuba, 04/2015 Nikolay Solyak 19

20. Survived particles at the entrance of magnet LCWS'15, Tsukuba, 04/2015 Nikolay Solyak 20 From all particles generated in cavity only ~4.5% of the particles will leave the cavity from each side. The rest will be dissipated inside the cavity. Due to cavity spacing (5.75 wavelength) dark current particle can be accelerated in CM in one direction only.

21. Tracking through Quadrupole Distribution of particles in cavity last in the string, just before Q2 Distribution of particles in Q2 ‣ Each track exiting cavity is randomly sampled 100 times with uniform distribution in azimuthal angle for tracking in quad; 200600 electrons are tracked 21 N Sum of FN weights Lost in cavity 358.1211 Exit to the left 10.5665 Exit to the right (to quad) 2006541.36 Quad 15 GeV Quad 125 GeV Quad 250 GeV N Sum of FN weights N N Lost in quad 2284213.2%11646163.6 %16210484.6 % Exit to the right 17775886.8%8413936.4 %3849415.4 %

22. Distribution of Losses in quad (125GeV) 22 Quad

23. Losses in quad (125GeV): energy distribution 23

24. R’ vs R at quad exit 24 Y vs X at quad exit Distribution at Quad exit (125 GeV) 36% of particles survived after Quad 36% of particles survived after Quad Energy ~ 700-800 MeV Energy ~ 700-800 MeV

25. Summary FE model for dark current studies is completed. Tracking is done for individual cavity and chain of downstream cavities and magnets. Three tasks are formulated for MARS simulations. ‣ Radiation from DC, lost in individual cavity (90% of generated particle) ‣ Radiation in ML caused by accelerated part of DC -Source of dark current – single cavity, located in worst position -All cavities contribute to DC uniformly (average) MARS geometry is built, cross-checked against lattice file and components dimensions. Few corrections identified and fixed. MARS simulations for task#1 are done, for task2&3 in progress 25

26. Back-up slides 26

27. Distribution of Losses in quad (15GeV) 27

28. Losses in quad (15 GeV): energy distribution 28

29. R’ vs R at quad exit 29 Y vs X at quad exit Color-distance before lost (mm) Distribution at Quad exit (15 GeV)

30. Quad energy 250 GeV 30 Distribution of Losses in quad (250 GeV)

31. Losses in quad(250GeV): energy distribution Quad energy 250 GeV 31

32. R’ vs R at quad exit 32 Y vs X at quad exit Losses in Quad (located at 250 GeV)